The demand for improvement of write speed on recordable optical discs is increasing for responding to larger capacity of digital information of recent years. Particularly the demand for higher speed is increasing for DVD media which have already been in widespread use. Although rewritable DVD-R/+R media applicable for up to 16× speed (with scan speed of 55.8 m/s and channel bit rate of 419 Mbps) have already been made commercially available, there has been a delay in speeding up of rewritable media and 6× speed (with scan speed of 20.94 m/s and channel bit rate of 157 Mbps) is the maximum for DVD-RW and 8× speed (with scan speed of 27.9 m/s and channel bit rate of 209 Mbps) is the maximum for DVD+RW. The main reason for this is the simultaneous pursuit of speeding up and reliability.
In general, rewritable optical discs are based on the reversible changes of recording layers using phase-change materials. Examples of rewritable phase-change optical discs include CD-RW (Compact Disc ReWritable), DVD-RW, DVD+RW, DVD-RAM (Digital Versatile Disc ReWritable), HD DVD-RW (High Density DVD ReWritable) and BD-RE (Blu-ray Disc ReWritable).
In particular, it is possible to perform recording, erasing and rewriting of information on these discs by forming amorphous marks in the crystalline phase by heat history of the recording layer material and crystallizing amorphous marks. The greatest benefit of this technique is that the phase changes can take place reversibly between crystalline and amorphous phases only by heat history and compatibility with read-only apparatuses is easily obtained because it does not require particular kind of optical systems.
Since it is necessary to control heat history, that is, quenching and gradual cooling, the maximum recording speed depends largely on the crystallization speed of recording layer materials. In other words, by using recording layer materials of high crystallization speed in order to correspond to high-speed recording, “gradual cooling” becomes possible even with a heat history in which “quenching” usually takes place. However, the recording layer materials of high crystallization speed tend to have low stability with time. The stability with time of the material in which high-speed phase changes occur with low energy is affected in the range between room temperature to operative temperature (approximately 60° C.).
It is generally known that the properties are degraded particularly when recording or overwriting was performed after long-term storage after non-recording or recording. The method in which recording tests are conducted after storing unrecorded discs is generally called “shelf”, and a method in which overwriting is performed after storing recorded discs is generally called “archival shelf”.
For the above reasons, standards relating to the archival shelf, “Overwrite Stability”, which assure reliability over time, are added to the standards for DVD+RW, 8× speed disc.
These archival-overwrites depend largely on the recording layer materials as stated above; however, it is possible to be improved by method for recording. In particular, it is possible to be improved by setting recording strategy (intensity modulation of light irradiated to the discs during recording) or by setting write power (intensity of light irradiated to the discs during recording) to its optimal.
Write power is usually determined by OPC (Optimum Power Control), the optimum write power control. This is a method in which test recordings are performed by drives on the test recording area (it is called “PCA”: “Power Calibration Area” or “Drive Test Area”) disposed on the disc to calculate the optimum write power.
When OPC is performed on a disc after long-term storage or shelf tests, however, it is difficult to set the calculated write power including a number of overwriting as the optimum condition because properties of PCA have also been changed. Moreover, there may be differences in powers calculated in the recorded area where time has passed since recording and unrecorded area where time has passed without recording.
The prior art literatures relating to the present invention are as described in the following.
The Nonpatent Literature 1 is US standards for CD-RW, in which 2T strategy applicable for high-speed recording is specified. Moreover, γ method for setting optimum write power is also described. However, there is no description for asymmetry in consideration of the shortest space right after the shortest mark used in the present invention.
In Nonpatent Literature 2, the “2T strategy”, which is used in the present invention, is specified in detail. The optical information recording medium, to which the method for recording of the present invention is applied, is assumed to be based on the standards. The γ method is described as OPC (Optimum Power Control).
In Nonpatent Literature 3, a method using beta (β), which is similar to asymmetry, is described as a procedure for OPC. However, there is no description relating to the use of different strategies between data portion and OPC portion.
Furthermore, a method and an apparatus for recording information in which test recording is performed by changing parameters of recording strategy with two types of patterns which differ from each other, and reproducing signal after test recording is evaluated in order to set optimum recording parameters are disclosed in Patent Literature 1. The similar method is also employed in the present invention, however, the method in Patent Literature 1 differs from that of the present invention because comparison using patterns only of 2mT (where “m” is a natural number) and patterns of (2m−1) T is the primary concern.
[Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2005-04800
[Nonpatent Literature 1] “Recordable Compact Disc Systems part III”, volume 3, version 1.1 (commonly called as “Orange Book”)
[Nonpatent Literature 2] “DVD+RW 4.7 Gbytes Basic Format Specification part 1”, volume 2, version 1.0 (Standards for DVD+RW applicable for 8× speed recording and rewriting)
[Nonpatent Literature 3] ECMA-349 [Data Interchange on 120 mm and 80 mm Optical Disk using +RFormat-Capacity: 4, 7 and 1, 46 Gbytes per Side (Recording speed up to 8× speed)] (Standards for DVD+R)